infrastructure for applying patches to Git source

[shiftkey]

There are some issues like #72 and #164 that require making changes to Git.

The rough idea is this:

• keep working from known stable Git versions as a submodule reference
• define a folder structure containing Git patches to apply to the Git submodule
• when running the build.sh step, ensure that patches for the given platform are applied before building, in the expected sort order (file format [number]-[patch-description].patch)
• after building, create a unique tag e.g. v2.20.1-dugite-{sha} where {sha} is the HEAD commit after applying the patches
• define some acceptance tests at the end of the build step that can verify the patches work as expected, e.g.

https://github.com/desktop/dugite-native/blob/dbeaba32b90a9e499c8ad518bb6257b060756c5d/script/build-ubuntu.sh#L126-L134

This will affect the generated Git version, because of how the GIT_VERSION file is generated by the Makefile. I might be the only one who notices or cares about this.